You are right. I was worried that receiving server would not recognize
"text/markdown" as a media type since it was registered recently in the
RFC. So if a server started to recognize that as media type, I thought they
should do that via SMTP extension.
My mail server doesn't support or understand any media types at all. MIME and
media types are all handled in the MUA.
Please review RFC 5598, the Internet Mail Architecture. The mail
server that advertises EHLO keywords is an MTA. MIME types are
handled in the MUA.
Also, practically, every modern MUA handles HTML now. If you want to use
markdown, adjust your MUA so it translates your markdown into text/html
and sends that. There are dozens of markdown->html translators so this
is just splicing together code that already exists.
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